Geofence tracking is the monitoring of movement of targets such as personal assets, vehicles or personnel within a defined geographic boundary. Geofence tracking is used to track and record the entry and exit of an assigned target (such as a vehicle) from a geographic boundary and to alert a user of the entry and/or exit activities of the assigned target.
A target's location within the geographic location may be established by position fixes. To obtain a highly accurate position fix, a mobile station device (e.g., portable device) may obtain SPS (Satellite Positioning System) pseudorange measurements and calculate its position relative to the geofence boundary. Alternatively, ground systems such as but not limited to Advanced Forward Link Trilateration (AFLT), Radio Frequency Identification (RFID), Bluetooth or Zigbee systems may be used. Achieving an accurate position fix using SPS pseudorange measurements requires the mobile station to perform complex calculations which can deplete the mobile station's battery power and use valuable network resources (e.g., geofence network resources, mobile station network resources, etc.). As used herein, SPS pseudorange measurements may be from a Global Positioning System (GPS), Galileo, Russian Global Navigation Satellite System (GLONASS), NAVSTAR, Global Navigation Satellite System (GNSS), a system that uses satellites from a combination of these systems, or any SPS developed in the future, each referred to generally herein as a Satellite Positioning System (SPS). As used herein, an SPS will also be understood to include analogous terrestrial ranging signal sources, such as pseudolites.